The energy demand in the world is increasing, and the forecast is a continued growth. Gas as an energy carrier has received increased attention recent years, and it is predicted that gas will become even more important. In order to transport gas over longer distances, liquefied natural gas, LNG, is often regarded as the best option, especially overseas.
Stranded gas or associated gas are gas sources which are “waste products” from oil production. These gas sources are today seldom utilized. They are commonly flared. With the increasing gas prices and more focus on the environment, it has become more economically viable and more politically important to utilize these sources. Many of these sources are offshore, and liquefaction on a floating production storage and offloading, FPSO, unit is in many cases the best option. FPSO's offer flexibility since they can be moved relatively easy to other sources. A challenge on the FPSO's is the space available. Furthermore, the weight of the equipment should be minimized, and the refrigerant should preferably be non-combustible.
An important issue for LNG production is the energy demand. High energy demand per kg produced LNG, i.e. specific energy consumption, makes it less profitable and less environmental friendly. The number of economically viable gas sources will be narrowed. Besides reducing operating cost, lower specific energy demand will also save investment cost, since the equipment will be smaller.
LNG production onshore does not have the same limitations with regard to weight and space but energy efficient LNG production is just as important. As the capacities of the plants gets larger, energy efficiency becomes more important.
Technology involving multi component refrigerant, MCR, often in cascades arrangements, is regarded as the most efficient technology for LNG production. It is commonly used in larger plants, base load plants, and to some extent in medium scale plants. Due to its complexity, MCR-technology is costly and control is slow. In addition, a gas make-up assembly is needed to ensure the correct composition of the MCR refrigerant. Another disadvantage is that the refrigerant is combustible which may be a problem, especially in offshore installations.
If a single component refrigeration technology using an inert gas, such as nitrogen, can be comparably energy efficient, it will represent a major improvement in terms of cost, compactness, weight, robustness, control, and safety. This technology can then be interesting to implement also in large scale plants.
U.S. Pat. Nos. 5,768,912 and 5,916,260 propose processes for LNG production based on nitrogen single refrigerant technology. The refrigerant is divided into at least two separate flows which are cooled and expanded in at least two separate expanders. Each of the flows are expanded down to the suction pressure of the compressor train, which is the lowest refrigerant pressure in the arrangement, thus using more energy than necessary.
U.S. Pat. No. 6,412,302 describes a LNG liquefaction assembly using two independent expander refrigeration cycles, one with methane or a mixture of hydrocarbons, and the other with nitrogen. Each cycle has one expander operating at different temperature levels. Each of the cycles can be controlled separately. Using two separate refrigerants will require two refrigerant buffer systems. Also using a flammable refrigerant implies restrictions or extra equipment.
Several patents are granted for MCR processes and apparatus using process gas as refrigerant, e.g. U.S. Pat. No. 7,225,636 and EP patent 1455152. Common for these are that heat absorption includes phase change of refrigerant, which inherently gives a more complex system. More equipment is needed and the control becomes complicated and sensitive.
There is a need for efficient processes based on an inert single component refrigerant. The present invention describes an energy efficient and compact LNG production assembly with a flexible control using an inert gas as refrigerant.